1. Field of the Invention
The present invention relates to a method and system for drying a substrate, and, more particularly, to a method and system for drying a substrate following exposure during immersion lithography.
2. Description of Related Art
In material processing methodologies, pattern etching comprises the application of a patterned mask of radiation-sensitive material, such as photoresist, to a thin film on an upper surface of a substrate, and transferring the mask pattern to the underlying thin film by etching. The patterning of the radiation-sensitive material generally involves coating an upper surface of the substrate with a thin film of radiation-sensitive material and then exposing the thin film of radiation-sensitive material to a radiation source through a reticle (and associated optics) using, for example, a photolithography system. Then a developing process is performed, during which the removal of the irradiated regions of the radiation-sensitive material occurs (as in the case of positive photoresist), or the removal of non-irradiated regions occurs (as in the case of negative resist) using a base developing solution, or solvent. The remaining radiation-sensitive material exposes the underlying substrate surface in a pattern that is ready to be etched into the surface. Photolithographic systems for performing the above-described material processing methodologies have become a mainstay of semiconductor device patterning for the last three decades, and are expected to continue in that role down to 65 nm resolution, and less.
The resolution (ro) of a photolithographic system determines the minimum size of devices that can be made using the system. Having a given lithographic constant k1, the resolution is given by the equationro=k1λ/NA,  (1)
where λ is the operational wavelength, and NA is the numerical aperture given by the equationNA=n·sin θo.  (2)
Angle θo is the angular semi-aperture of the system, and n is the index of refraction of the material filling the space between the system and the substrate to be patterned.
Following equation (1), conventional methods of resolution improvement have lead to three trends in photolithographic technology: (1) reduction in wavelength λ from mercury g-line (436 nm) to the 193 nm excimer laser, and further to 157 nm and the still developing extreme-ultraviolet (EUV) wavelengths; (2) implementation of resolution enhancement techniques (RETs) such as phase-shifting masks, and off-axis illumination that have lead to a reduction in the lithographic constant k1 from approximately a value of 0.6 to values approaching 0.4; and (3) increases in the numerical aperture (NA) via improvements in optical designs, manufacturing techniques, and metrology. These latter improvements have created increases in NA from approximately 0.35 to values greater than 0.75, with 0.85 expected in the next few years. However, as can be seen in equation (2), for conventional free-space optical systems (i.e., n=1), there is a theoretical limit bounding NA to values of one or less.
Immersion lithography provides another possibility for increasing the NA of an optical system, such as a lithographic system. In immersion lithography, a substrate is immersed in a high-index of refraction fluid (also referred to as an immersion medium), such that the space between a final optical element and the substrate is filled with a high-index fluid (i.e., n>1). Accordingly, immersion provides the possibility of increasing resolution by increasing the NA beyond the free-space theoretical limit of one (see equations (1), and (2)).
Due to the inherently lower cost, relatively easy implementation to current exposure tools, and high potential to reach very high resolution with reasonable process latitude, liquid immersion lithography has emerged as a very promising candidate for semiconductor patterning technology down to 65 nm, 45 nm, and beyond. However, immersion lithography technology still faces numerous challenges including selection of an immersion fluid that is compatible with current and future photoresists yet free of optical defects (such as micro-bubbles) and sufficiently transparent, and selection of an immersion process that facilitates integration with existing exposure systems and track systems. Moreover, problems associated with introducing a liquid to the photolithography process must be identified, and new system components and methods for solving or reducing such problems must be developed.